This invention relates to the protection of induction motors, and in particular it relates to apparatus and method for the protection of rotors in squirrel cage induction motors.
It is known to provide protection of various types for alternating current (A.C.) motors. One well known type of protection is the overload relay which disconnects the motor from the power source when the current exceeds a predetermined level for a certain time. Another type of protection apparatus uses a resistor-capacitor network to simulate the motor stator where the capacitor represents the thermal capacity of the conductors and an associated resistor represents the thermal resistance of the insulation. The capacitor is charged on one side by a current representing heat generated by copper loss in the conductor, and it is charged on the other side by a current representing the temperature at the outer surface of the insulation determined from a temperature sensor. The network can thus be used as an analog of the heat in the winding and can be used to disconnect the motor from the supply to prevent damage to the insulation. An example of this type of protection is described in Canadian Patent No. 983,094--Boothman et al., issued Feb. 3, 1976 and granted to Canadian General Electric Company Limited.
Another type of apparatus for protecting motors is described in U.S. patent application Ser. No. 488,449 by D. R. Boothman, et al., filed Apr. 25, 1983 (corresponding to Canadian Patent Application Serial No. 402,483--Boothman et al., filed May 7, 1982) and assigned to Canadian General Electric Company Limited. This apparatus provides a digital thermal model of the motor. The thermal model is, however, primarily a thermal model of the stator. Any thermal characteristics of the rotor are not directly considered, although some of the thermal effects of the rotor may to some extent appear in the stator thermal characteristics. In the apparatus described in this co-pending application, a sensor and an analog to digital converter store in a first register a value representing heat generated in the conductors. A second register holds values representing core temperature. Values are determined for heat transferred away from the conductors and also for heat transferred into the core. One is subtracted from the first register and the other is added to the second register. Another value is determined for heat lost by the core material, and this value is subtracted from the second register. Thus the first and second registers hold values representing conductor temperature and core temperature respectively, and these can be used to interrupt the supply of power to the motor when a predetermined level is exceeded in either register.
It is, of course, known that phase loss and phase reversal in the supply of power to a three phase motor can cause severe problems with generation of heat. Many systems have been designed to overcome this. For example, U.S. Pat. No. 3,743,889 by LOPEZ-BATIZ, issued July 3, 1973 and assigned to Hatch Incorporated, describes such a system. However, none of the prior systems are primarily intended for protection of the rotor and they do not use a thermal model of the rotor.
The rotor of a squirrel cage motor is in many cases the limiting consideration in the design and it is important to provide suitable protection for it. A squirrel cage motor should have a high starting torque but maintain a high efficiency at normal operating speeds. In order to achieve the desired characteristics, the designer of a squirrel cage motor intentionally designs the rotor to be frequency sensitive. The squirrel cage conductor bar may be made deep and narrow (relatively large in the radial dimension extending inwardly from the rotor periphery, and relatively narrow in the peripheral or circumferential direction). A more detailed description of the design procedures for deep rotor bars can be found in books, such as, for example, "Induction Machines", Philip L. Alger, Second Edition, Gordon and Breach Science Publishers, about page 265. When a conductor bar is made deep and narrow, the current will be crowded towards the top (i.e., towards the peripheral end) during starting when the slip frequency is high thereby increasing the effective resistance. At operating speed when the slip frequency is low, the current tends to be uniformly distributed giving a low resistance. Indeed, it is also well known to provide a necked portion in the rotor bar to assist in dividing the current during starting from the current at normal operating speeds, although this is not essential as the desired effect appears even in a symmetrical conductor bar.
In other words, during starting, the rotor is designed to have the current in a rotor bar constrained to a smaller cross-section than when running at normal speeds. This provides the desired higher starting torque while retaining a high efficiency at running speeds. When the rotor is designed to be frequency sensitive to obtain desired starting/running characteristics, it is, of course, also frequency sensitive to other frequency related conditions, such as for example, discrepancies in the symmetry of the supply, improper phase sequence and harmonic content. This is in addition to phase loss and phase reversal problems which, in effect, cause positive and/or negative sequence frequency components of current which may be at higher frequencies.
The use of symmetrical positive and negative sequence current components in analyzing electrical apparatus is well known. Dr. Charles L. Fortescue described this in his 1918 paper entitled "Method of Symmetrical Co-ordinates Applied to the Solution of Polyphase Networks", Trans AIEE, about page 1027. Further references and further description may be found in the Standard Handbook for Electrical Engineers, Editor-in-Chief Donald G. Fink, Eleventh Edition, beginning about 2-48. It is believed no detailed explanation is necessary herein.
If one phase of a three phase motor were to be open during starting, high currents would be generated in the stator winding which would result in overheating. The overheating would also occur in a running motor with the rate of heating depending on the load. Similarly, a phase reversal may cause overheating of the stator. Equipment is available to detect a phase loss and a negative phase sequence and to interrupt a power supply if predetermined limits are exceeded. For example, a negative phase sequence time overcurrent relay (type INC77B) is available from General Electric Company. The aforementioned U.S. Pat. No. 3,743,889 describes circuitry capable of detecting positive and negative voltage sequence. In addition, a paper entitled "A Microcomputer Based Symmetrical Component Distance Relay", by A. G. Phadke, T. Hlibka, M. Ibrahim and M. G. Adamiak, in 1979 Power Industry Computer Applications Conference, describes yet another way of obtaining an evaluation of positive and negative voltage sequences. It does this using a microcomputer and in addition it readily provides an evaluation of positive and negative voltage sequences of any desired harmonics.
When a squirrel cage motor is running and the power supply has only a positive sequence, the current in the rotor bars is at slip frequency. However, any unbalance will introduce a negative sequence current component which, because of the rotational speed of the motor, will be at almost twice the line frequency. Similarly, any harmonics which appear in the supply will not only affect the stator but will appear in the rotor and, if there is a negative sequence component, will appear as a multiple of the harmonic frequency. Because the rotor is designed to be frequency sensitive, the greater the frequency, the greater will be the overheating. Thus, the rotor often is affected more than the stator by any irregularities in the supply.